1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copier, a printer or a facsimile machine typically includes a photoconductive drum used as an image bearing member. The image forming apparatus also includes a charger configured to apply a predetermined voltage to uniformly charge a circumferential surface of the photoconductive drum and an exposure device configured to irradiate the charged circumferential surface of the photoconductive drum with a laser beam. The irradiation of the laser beam from the exposure device partially causes optical attenuation in a potential on the circumferential surface of the photoconductive drum to form an electrostatic latent image in conformity with a document image on the photoconductive drum.
The image forming apparatus also includes a developing device configured to supply toner to the photoconductive drum on which the electrostatic latent image is formed. The toner from the developing device adheres to the circumferential surface of the photoconductive drum to develop the electrostatic latent image based on a relationship among parameters such as a surface potential of the electrostatic latent image, a charged amount of the toner itself and a bias voltage of the developing device.
The image forming apparatus further includes a transfer element such as a transfer roller near the photoconductive drum. A toner image resulting from the development of the electrostatic latent image is transferred to a sheet passing between the photoconductive drum and the transfer roller. The transfer element applies a bias voltage (transfer voltage) to electrostatically transfer the toner image from the photoconductive drum to the sheet.
The electrostatic transfer of the toner image from the photoconductive drum to the sheet causes the sheet itself to be charged. The charged sheet is likely to adhere to the circumferential surface of the photoconductive drum.
A typical image forming apparatus includes a separation electrode configured to electrically neutralize a sheet after transfer of a toner image. A separation voltage (bias voltage having a polarity opposite to that of a transfer voltage) applied to the separation electrode induces a discharge between the sheet and the separation electrode to reduce a charged amount of the sheet, so that the separation voltage facilitates to separate the sheet from the photoconductive drum.
The aforementioned image forming technology (technology for directly transferring a toner image from a photoconductive drum (image bearing member) to a sheet) is typically applied to image forming apparatuses configured to perform monochrome printing.
A tandem image forming apparatus configured to work for color printing typically performs a primary transfer process for transferring a toner image to an intermediate transfer member and a secondary transfer process for transferring the toner image from the intermediate transfer member to a sheet. The image forming apparatus includes a few aligned image forming portions. The image forming portions form different toner images in hue, respectively. Each of the image forming portions includes a photoconductive drum, on which a toner image is to be formed.
The image forming apparatus also includes transfer elements corresponding to the photoconductive drums of the image forming portions, respectively. In the primary transfer process, bias voltages are applied to the transfer elements. As a result, toner images on the photoconductive drums are successively transferred to the intermediate transfer member, respectively, and superimposed thereon.
The image forming apparatus also includes another transfer element configured to transfer the toner images from the intermediate transfer members to a sheet. In the secondary transfer process, a bias voltage (transfer voltage) is applied to this transfer element. As a result, the toner images superimposed on the intermediate transfer member are transferred to the sheet. The sheet charged due to the secondary transfer process is likely to adhere to the intermediate transfer member.
The typical image forming apparatus configured to perform color printing includes a separation electrode configured to electrically neutralize a sheet after transfer of a toner image thereto similarly to the above image forming apparatus configured to perform monochrome printing. A separation voltage (bias voltage having a polarity opposite to that of a transfer voltage) applied to the separation electrode induces a discharge between the sheet and the separation electrode to reduce a charged amount of the sheet, so that the separation voltage facilitates to separate the sheet from the intermediate transfer member.
Incomplete transfer (white dot phenomenon) in which a transferred toner image partially misses is known as a drawback of the aforementioned image forming apparatuses. As a result of the incomplete transfer of the toner image, a user sees dispersed small dot areas where a color of the sheet appears.
The incomplete transfer of a toner image is typically likely to occur when the toner image is transferred to a sheet with a larger resistance value under a low-temperature and low-humidity environment. Particularly, when a toner image is transferred to a second surface opposite to a first surface bearing a previously formed and fixed toner image, the incomplete transfer of the toner image is likely to occur.
A transfer voltage is generally applied to the transfer element configured to transfer a toner image to a sheet under a constant current control in which a constant current is generated. A larger absolute value of a transfer voltage needs to be applied to the transfer element immediately before or during passage of a sheet with a larger resistance value between an image bearing member and an transfer element. The application of the larger absolute value of the transfer voltage is likely to induce a discharge of the transfer element or the sheet. The discharge of the transfer element or the sheet produces non-charged toner or toner charged with an opposite polarity. The non-charged toner or toner charged with an opposite polarity is not transferred to the sheet, which results in the aforementioned incomplete transfer. The color of the sheet itself appears in tiny parts where the toner has not been transferred.
A photoconductive drum including amorphous silicon has a larger capacitance than organic photoconductors (OPCs). Thus, the photoconductive drum including amorphous silicon requires application of a larger absolute value of a transfer voltage. Therefore, incomplete transfer of a toner image is more likely to occur in the transfer of the toner image from the photoconductive drum including amorphous silicon.
The toner on the intermediate transfer member of the aforementioned tandem image forming apparatus is exposed to transfer voltages generated between the photoconductive drums and the transfer elements for the primary transfer a few times to be further charged. Accordingly, the toner on the intermediate transfer member requires application of a larger absolute value of a transfer voltage to the transfer element for the secondary transfer when being transferred to a sheet. Therefore, tandem image forming apparatuses are more likely to cause incomplete transfer of a toner image.